Doubling the life support, almost doubling the crew space, carrying lots of extra mass - the recent question about direct ascent made me ponder this: the fundamental role of the Command Module was to provide transfer both ways and reentry capabilities to the Apollo missions. None of these requires a crewed module. Why wasn't the 'command module' just an umnanned tug?

Let's imagine this: a single crewed cabin, with lunar descent and ascent modules (both detachable once used up) that can undock/redock to a heat shield, which in order is attached to a module with high ISp low thrust propulsion for transfer burns (with radio relay capacity too). There should be huge mass savings on not making that module crewed. They could be redirected to making a larger lander, or just reduce the cost. Additionally, docking with crew transfer capability is way more complex than without it.

2 Answers
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The primary function of the command module was to keep three crewmen alive and allow them to survive re-entry. Those requirements weren't negotiable.

The Soyuz spacecraft to some extent divides those functions into two modules, and so its re-entry module is more compact than the Apollo CM (Command Module), but this comes with an overall complexity penalty.

The point of the LOR (Lunar Orbit Rendezvous) strategy is that it allowed the lunar landing delta-v cost to be paid by a smaller craft. The LM (Lunar Module) cabin supported about 8 man-days of operation in very cramped quarters; the CM supported 36 man-days in relative luxury. Landing the CM would require a much larger descent-ascent system than the LM — greatly exceeding the mass of the "redundant" LM cabin.

In essence, you'd be calling for a small optimization of the direct ascent strategy — leaving the TEI (Trans-Earth Injection) fuel and possibly the heat shield in lunar orbit for the landing, at the cost of requiring a rendezvous (like LOR).

The fueled mass of the Apollo LM (15 tons) is more than 7.5 times the mass of the cabin (ascent stage, dry, is 2.1 tons, and consists of the cabin, ascent engine, tankage, and various additional equipment).

The Apollo CM, less the heat shield, is about 4.7 tons. Even stripped down, say to 4 tons, you'd need 30 tons for a complete lunar descent and ascent at the same mass ratio as the LM. So to save the mass of the 2-ton LM cabin you have to incur a 15-ton hit in the LM.

If you define a two-man mission instead, the CM could be scaled down to perhaps 3 tons, which leads to something like a 5 ton penalty. Remember, though, the Apollo CSM (Command/Service Module) was conceived as a general purpose 3-man spacecraft to ferry crews to space stations and so forth, not solely for moon landings. (After the Apollo lunar program, the spacecraft was used for Skylab missions and the Apollo-Soyuz Test Project).

A single cabin solution also removes the lifeboat capability that saved the lives of the Apollo 13 crew, though that wasn't a primary driver of the LOR architecture.

To undock/redock to a heat shield to the crew cabin would be very difficult to impossible. A main part of the heat shield should be removed and reinserted between the crew cabin and the ascent/descent stage as well as the service module. But there are a lot of connections (electrical for sensors and actors, water, cooling liquids and oxygen) between the crew cabin and the other modules.

The heat shield should be left in orbit around the Moon or the Earth. If the heat shield is left in Earth's orbit, extra fuel is necessary for transition from return trajectory to orbit.

The heat shield should not only be docked, it must be mounted with airtight seals to prevent entry of the hot high pressure gas from reentry. The heat shield should fit to all attitude control thrusters and the cabin windows. Of course the parachutes needed for landing must be protected by the heat shield too and the heat shield should allow the ejection of the parachutes.

$\begingroup$I believe the proposal is just for the thick, heavy ablative heat shield on the base of the capsule, not the entire thermal protection solution.$\endgroup$
– Russell BorogoveOct 24 '18 at 19:43

$\begingroup$@RussellBorogove: But a cone shape crew capsule like the Apollo Command Module needed not only a thick heat shield at the base of the capsule but also a thinner heat shield for the cone part. Without a heat shield at the cone part only a steep high G-force reentry with more heat load is possible. For a skip reentry with lower G-force and heat load a heat shield at the cone part is necessary too.$\endgroup$
– UweOct 24 '18 at 19:53

$\begingroup$I interpret OP's proposal as keeping the light thin upper part of the heat shield permanently attached, and making the heavy base shield detachable. In casual use, "Apollo CM's heat shield" nearly always refers to the base portion rather than the entire TPS.$\endgroup$
– Russell BorogoveOct 24 '18 at 21:00